Remove the encryption file, replace the encrypted fonts with non-encrypted versions and done.

One important note, if this eBook is going to be read on an eInk Reader, do not use Myriad Pro Light. It's too light for eInk. It will look terrible. If this is the body font (regardless of which version), it's sans-serif and again, won't look good.

You cannot use font-weight: 300. It won't work. You can use font-weight: bold if you've also embedded the bold version.

One thing you can do to not have to encrypt the fonts is to have Calibre subset the font and use the subsetted versions of the font. Not only does it make the eBook smaller in size, it makes the fonts useless outside of that specific eBook.